Multiple tunnel junction thermotunnel device on the basis of ballistic electrons

ABSTRACT

The present invention is a tunnel diode, in which the space between the emitter electrode and the collector electrode is occupied by a porous material which has a thickness less then the free mean free path of an electron in the porous material. The present invention also includes heat pumping and power generation devices comprising the tunnel diode.

TECHNICAL FIELD

The present invention relates to tunnel junction diodes. It also relatesto devices for heat pumping and electrical energy generation,particularly to thermotunnel devices. The present invention utilizesballistic electrons and quantum mechanical effects that work only forballistic electrons.

BACKGROUND ART

In order to operate thermoelectric heat pumps and energy converters in ahigh efficiency regime one needs to select electrons by energy. In thecase where mostly only high-energy electrons are used for heattransport, efficiency is considerably increased.

In U.S. Pat. No. 3,169,200, an energy converter comprising multipletunnel junctions connected in series is described. Tunnel junctionscomprise two metal electrodes separated by a thin insulator layer. Whena thermal gradient is maintained across the device, thermally excitedelectrons tunnel trough the tunnel junctions and generate outputvoltage. One disadvantage of such a converter is that it does not have ahigh enough selectivity for electrons by energy because it does notutilize ballistic electrons. Another disadvantage results from thelosses due to thermal conduction. Tunnel barriers are very thin (of theorder of 10 Angstroms) and thermal backflow in a particular tunneljunction is very high because of the use of a solid insulator layerbetween metallic electrodes. Because of this, 10⁵ junctions need to beconnected in series to reduce thermal backflow and obtain efficient heatto electrical energy transfer. Fabrication of such a number of tunneljunctions connected in series appears to be practically impossible.

There remains a need in the art therefore for a device having fewerelements, which is easier to fabricate, and in which losses due tothermal conduction are further reduced.

Previously we have described a thermotunnel device that could be usedboth for heat pumping and electrical energy generation (U.S. Pat. No.6,417,060; WO99/13562). Such a thermotunnel device comprises two metalelectrodes separated by thin vacuum gap, as is shown in FIG. 1.Electrons tunnel from a hot emitter 100 to a cold collector 102 througha vacuum gap 104. FIG. 1 also shows the energy diagram for the device.Here E_(f) is Fermi energy of emitter and E_(v) vacuum energy level.Consider two electrons sitting on different quantum energy levels in theemitter: one electron 106 has a higher energy, and the other electron108 has a lower energy. Let the probabilities of tunnelling be ρ₁ and ρ₂respectively, as shown. The probability of tunnelling is greater for theelectron having the higher energy ρ₁>ρ₂. Thus the single tunnel barrierselects electrons by energy. However this selection process is notenough by itself because the density of energy states decreasesexponentially when the energy is increased (depending on the workfunction of the metal and its temperature). Overall, the tunnellingcurrent from the interval dE is the probability of tunnelling multipliedby the density of energy states. Consequently the contribution of lowenergy electrons to the tunnelling current is still considerable.

FIG. 2 shows an emitter electrode 100, a collector electrode 102 and anumber of islands 210, 212 disposed between them. The islands arepreferably metallic. Each of the islands has a thickness b. For the sakeof simplicity, two such islands are shown, but any number n may beutilized. The thickness, b, of the islands is chosen such that theirtotal thickness is less than the mean free path of an electron in theparticular material, L. Under these conditions, for the case whenelectrons are ballistic, (n−1)b<L, and the electron can travel throughmany tunnel junctions without entering into thermal equilibrium with theelectron gas and lattice in the metallic islands.

Thus the thickness of the islands and number of the islands is lowenough that an electron can travel through such a system withoutinteraction with lattice inside the islands. For such a system, theprobabilities of tunneling for two ballistic electrons 106 and 108sitting on different quantum energy levels are ρ₁ ^(n) and ρ₂ ^(n)correspondently. The ratio of probabilities of tunneling will be ρ₁^(n)/ρ₂ ^(n)=(ρ₁/ρ₂)^(n). Thus the ratio of probabilities of tunnelingfor multiple junctions is n-th degree of the ratio of probabilities ofthe single junction shown in FIG. 1. Given formula is true only in thecase the same electron tunnels through all of the tunnel barriers(ballistic tunneling). It is obvious that this ratio increases verysharply as the number of tunnel junctions connected in series isincreased. This means that selectivity is greatly increased in multipletunnel junctions connected in series, and a thermotunnel device based onsuch multiple tunnel junctions will have a high efficiency.

Such multiple tunnel junctions are very difficult to fabricate. Whilstit is possible to achieve a thin vacuum gap over large areas for asingle tunnel junction, duplicating it and connecting junctions inseries is not possible for current nano-engineering techniques. This isbecause very thin islands are needed to obtain ballistic transportregime: the integrated width of all the islands should be less than meanfree path of the electron in the given material (mean free path of theelectron is in the range of 1-100 nm for metals). Thin films of suchthickness are almost impossible to fabricate; in addition it remainsunclear how a vacuum gap between them could be maintained and stabilizedunder the influence of the electrostatic forces between islands.

One practical solution is porous,materials and particularly poroussilicon that has pore size of the order of nanometers. Such material hasbeen used for photoluminescence device fabrication (Nakajima et al.(2002) Appl. Phys. Lett. 81:2472-2474). The device is composed of asemitransparent top electrode, a thin film of fluorescent material, anano-crystalline porous silicon layer, an n-type silicon wafer, and anohmic back contact. When a positive dc voltage is applied to the topelectrode with respect to the substrate, electrons injected into thenano-crystalline porous silicon layer are accelerated via multipletunneling through interconnected silicon nano-crystallites, and reachthe outer surface as energetic hot or quasi-ballistic electrons.Experimental results obtained from porous silicon show clear filteringof electrons by energies.

Another work (Ozaki et al. (1995) Jap. J. Appl. Phys. 24:946-949)investigates the nature of electron filtering mechanism in porousmaterial and experimental results showed that tunneling is responsiblefor filtering electrons by energy.

DISCLOSES OF INVENTION

From the foregoing, it may be appreciated that a need has arisen for aprocess and a device in which the benefits of multiple tunnel junctionscan be harnessed for increasing selectivity of tunnelling.

Here we disclose a solution that uses porous materials as multiplevacuum tunnel barriers to increase selectivity of tunnelling. We suggestthe use of such a material as an electron filter in a thermoelectricdevice.

The present invention is a tunnel diode, in which the space between theemitter electrode and the collector electrode is occupied by a porousmaterial which has a thickness less then the free mean free path of anelectron in the porous material. The present invention also includesheat pumping and power generation devices comprising the tunnel diode.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete explanation of the present invention and thetechnical advantages thereof, reference is now made to the followingdescription and the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of a thermotunnel device of theprior art.

FIG. 2 is a diagrammatic representation of a multiple tunnel junctiondevice on the basis of ballistic electrons.

FIG. 3 is a diagrammatic representation of a multiple tunnel junctionthermoelectric device of the present invention.

FIG. 4 is a diagrammatic representation of an apparatus for theconversion of energy of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention and their technical advantages maybe better understood by referring to FIG. 3.

FIG. 3 shows an emitter electrode 100, a collector electrode 102 and aporous layer 300 disposed between them. The thickness of the porousmaterial is selected so that it is less than mean free path of theelectron in given material and for given pore density. Typically, thethickness of the porous material is 1-100 nm. In a preferred embodiment,the porous layer is porous silicon. In a further preferred embodiment,the porous silicon is doped to alter the mean free path of the electron.Electrode 100 is thermally connected to a heat source 302 and electrode102 is thermally connected to heat sink 304. Electrons in electrode 100are excited to high energies by the heat source, and high-energyelectrons tunnel to electrode 102, producing a voltage drop between thetwo electrodes. As the electrons move from one electrode to the other,they tunnel through the many pores inside the porous material. Becauseof this multiple tunneling, electrons are sharply selected according totheir energy, which means that only electrons having the highestenergies can take part in heat transport. Thus the efficiency of energyconversion is increased by the filtering effect of porous materialrelative to a device utilizing a single tunnel junction.

Referring now to FIG. 4, which shows in diagrammatic form an apparatusfor the conversion of energy, a source of thermal energy 302 isconnected via a thermal interface 400 to an emitter electrode 100. Aheat sink 304 is connected via a thermal interface 402 to a collectorelectrode 102. A porous material 300 is disposed between the emitterelectrode and the collector electrode as shown. An electrical circuit404 connects the two electrodes.

For power generation, an electrical load 406 forms part of circuit 404.The source of thermal energy may be solar, or may be from the combustionof fuel, or may be waste heat. The source of thermal energy promotes theflow of electrons from emitter to collector through the electrical loadvia the external circuit.

For the conversion of electrical energy to heat pumping capacity, anelectrical power supply 406 forms part of circuit 404. The electricalpower supply applies a voltage bias to the electrodes, and causeselectrons to flow from the emitter electrode to the collector electrode,resulting in a transfer of thermal energy from the emitter to thecollector. The source of thermal energy may be cooler than the heatsink.

It might be considered that the heat conductance of the porous layercould deleteriously influence the efficiency of such a device because ofheat backflow. However heat conductivity of porous silicon has beeninvestigated and it is found that porous material has a very low heatconductivity (Zeng et al. (1995) Transactions of ASME Journal of HeatTransfer 117:758-761). Porous silicon is therefore used for heatinsulation in some experimental devices. It is believed that the mainmechanism responsible for the low heat conductivity is due to a changein the physics of heat transfer, resulting from the pore dimensionsbeing less than mean free path of atmospheric gas molecules.

INDUSTRIAL APPLICABILITY

The present invention may be applied to a variety of tunnel junctionapplications, including heat pumping and power generation.

1. A tunnel diode comprising: (a) an emitter electrode, in contact with(b) a porous material, in contact with (c) a collector electrode whereinsaid porous material has a thickness which is less then the free meanfree path of an electron in said porous material.
 2. The tunnel diode ofclaim 1 in which said porous material comprises porous silicon.
 3. Thetunnel diode of claim 1 in which said porous material comprises dopedporous silicon.
 4. The tunnel diode of claim 1 in which said thicknessis in the range of 1 to 100 nm.
 5. The tunnel diode of claim 1additionally comprising a heat source in contact with said emitterelectrode.
 6. The tunnel diode of claim 1 additionally comprising a heatsink in contact with said collector electrode.
 7. Apparatus for theconversion of energy comprising: (a) a source of energy; (b) an emitterelectrode connected to said source of energy; (c) a collector electrode;(d) a porous material disposed between said emitter electrode and saidcollector electrode; (e) an electrical circuit connecting saidelectrodes; and wherein said porous material has a thickness which isless then the free mean free path of an electron in said porousmaterial.
 8. The apparatus of claim 7 in which said porous materialcomprises porous silicon.
 9. The apparatus of claim 7 in which saidporous material comprises doped porous silicon.
 10. The apparatus ofclaim 7 in which said thickness is in the range of 1 to 100 nm.
 11. Theapparatus of claim 7, wherein the conversion of energy is the conversionof thermal energy to electrical energy, wherein said source of energycomprises a source of thermal energy, and wherein said apparatus furthercomprises: a) a first thermal interface thermally connecting said sourceof energy to said emitter electrode; b) a second thermal interfacethermally connecting a heat sink means to said collector electrode; c)an electrical load, electrically connected by said circuit between saidcollector electrode and said emitter electrode.
 12. The apparatus ofclaim 11 wherein said source of thermal energy is of solar origin. 13.The apparatus of claim 7, wherein the conversion of energy is theconversion of electrical energy to heat pumping capacity, and whereinsaid apparatus further comprises: a) a heat source and a heat sink,wherein said heat source is thermally connected to said emitterelectrode and said heat sink is thermally connected to said collectorelectrode, and, b) an electrical power supply, electrically connected bysaid circuit between said collector electrode and said emitter electrodefor applying a voltage bias to said electrodes, said electrical powersupply providing said energy source.
 14. The apparatus of claim 13wherein said heat source may be cooler than said heat sink.